The nuclear envelope is a distinctive feature of all eukaryotic cells. It is composed of three major elements: 1) a parallel double membrane system in which the inner and outer nuclear membranes are continuous at circular discontinuities in the envelope known as "pores", 2) a morphologically well-defined proteinaceous material situated within the pores known as the nuclear pore complex, and 3) a distinct protein- aceous lamina of variable dimensions and ultrastructural arrangements which is located immediately beneath the inner nuclear membrane and commonly called the fibrous lamina or dense lamella. A primary goal of this investigation is the isolation, initially from rat liver, of the two nonmembranous components of the envelope: the nuclear pore complex and the lamina. The isolated components will be characterized with respect to their chemical composition, a comparison of their ultrastructure before and after isolation, and with respect to functional activities. The biogenesis and function of the pore complex and peripheral lamina as organelles will be studied in synchronized mammalian cell culture or in rat liver.